Purification of nitrogen trifluoride by pressure swing absorption

Abstract

A pressure-swing adsorption process for producing a purified nitrogen trifluoride (NF 3 ) product gas from a feed gas contaminated with carbon tetrafluoride (CF 4 ) using a column packed with an adsorbent that exhibits selectivity for NF 3 over CF 4 . The process separates the feed gas into a first product gas having a greater concentration of CF 4 than the feed gas and a second product gas having a lesser concentration of CF 4 without the use of an inert carrier gas while recovering essentially all of the NF 3 . The process includes filling the column with the feed gas until the column reaches a first pressure, removing the first product gas, and then removing the second product gas, where after the second product gas is removed the column is at a second pressure. A rinse gas of NF 3 and CF 4 may be used to remove the first product gas or the second product gas.

What is claimed is: 
     
       1. A method for purifying nitrogen trifluoride (NF 3 ), the method comprising:
 filling an adsorption column with a first volume of a feed gas consisting essentially of NF 3  and carbon tetrafluoride (CF 4 ) until the adsorption column reaches a first pressure greater than an initial pressure of the adsorption column, wherein the adsorption column includes an adsorbent material that selectively adsorbs NF 3  over CF 4 , 
 whereby the adsorbent material adsorbs a portion of the feed gas having a greater concentration of NF 3  than an unadsorbed portion; 
 removing a first product gas having a greater concentration of CF 4  than the feed gas from the adsorption column; and, 
 after removing the first product gas, removing a second product gas having a lesser concentration of CF 4  than the feed gas from the adsorption column, wherein the column has a second pressure less than the first pressure after removing the second product gas. 
 
     
     
       2. The method of  claim 1 , wherein essentially all of the NF 3  of the feed gas is removed as a part of the first product gas, as a part of the second product gas, or remains in the adsorption column. 
     
     
       3. The method of  claim 1 , wherein:
 removing the first product gas from the adsorption column comprises maintaining essentially the first pressure in the adsorption column by flowing a rinse gas consisting essentially of NF 3  and CF 4  into the adsorption column while removing the first product; and 
 removing the second product gas comprises reducing the pressure within the adsorption column to the second pressure. 
 
     
     
       4. The method of  claim 3 , wherein the rinse gas comprises a second volume of the feed gas or a volume of the second product gas. 
     
     
       5. The method of  claim 1 , wherein:
 removing the first product gas from the adsorption column comprises reducing the pressure within the adsorption column to a third pressure between the first pressure and the second pressure; and 
 removing the second product gas comprises reducing the pressure within the adsorption column from the third pressure to the second pressure. 
 
     
     
       6. The method of  claim 5 , further comprising, after the adsorption column reaches the third pressure, continuing to remove the first product gas while maintaining the third pressure in the adsorption column by flowing a rinse gas consisting essentially of NF 3  and CF 4  into the adsorption column. 
     
     
       7. The method of  claim 6 , wherein the rinse gas comprises a second volume of the feed gas or a volume of the second product gas. 
     
     
       8. The method of  claim 1 , wherein an inert carrier gas is not flowed into the adsorption column during any step of the method. 
     
     
       9. The method of  claim 1 , wherein the adsorbent material comprises an equilibrium-selective material. 
     
     
       10. The method of  claim 9 , where the equilibrium-selective material comprise a zeolite selected from the group consisting of zeolites of types LTA (Linde type A), FAU (Faujasite), CHA (Chabazite), and ERI (Erionite). 
     
     
       11. The method of  claim 10 , wherein the equilibrium-selective material comprises a mixture of primarily CHA zeolite with a smaller amount of erionite. 
     
     
       12. The method of  claim 1 , wherein the adsorbent material is a material kinetically selective for NF 3  having pore openings ranging from approximately 3.6 angstroms to approximately 4.6 angstroms. 
     
     
       13. A method for separating a mixture comprising NF 3  and CF 4 , the method comprising:
 providing an adsorption column packed with an adsorbent material that preferentially adsorbs NF 3  over CF 4 , wherein the adsorption column comprises a feed end and, a product end opposite the feed end, wherein the feed end and the product end are initially closed; 
 opening the feed end of the adsorption column; 
 flowing a first volume of a feed gas comprising NF 3  and CF 4  into the feed end of the adsorption column until the adsorption column reaches a first pressure, whereby a fraction of the NF 3  of the first volume of the feed gas is adsorbed by the adsorption material; 
 opening the product end of the adsorption column; 
 flowing a second volume of the feed gas into the feed end of the adsorption column to maintain the first pressure within the adsorption column, whereby a first product having a CF 4  concentration greater than the CF 4  concentration of the feed gas exits the product end of the adsorption column; and 
 closing the feed end of the adsorption column, whereby a second product having a CF 4  concentration less than the CF 4  concentration of the feed gas exits the product end of the adsorption column until the adsorption column reaches a second pressure less than the first pressure. 
 
     
     
       14. The method of  claim 13 , further comprising:
 opening the feed end of the adsorption column and closing the product end of the adsorption column; 
 flowing a second volume of the feed gas into the feed end of the adsorption column until the adsorption column reaches the first pressure, whereby a fraction of the NF 3  of the second volume of the feed gas is adsorbed by the adsorption material; 
 opening the product end of the adsorption column; 
 flowing a portion of the second product into the feed end of the adsorption column to maintain the first pressure within the adsorption column, whereby a third product having a CF 4  concentration greater than the CF 4  concentration of the feed gas exits the product end of the adsorption column; and 
 closing the feed end of the adsorption column, whereby a fourth product having a CF 4  concentration less than the CF 4  concentration of the feed gas exits the product end of the adsorption column until the adsorption column reaches the second pressure. 
 
     
     
       15. The method of  claim 14 , wherein the method does not comprise using a substantial volume of an inert carrier gas. 
     
     
       16. The method of  claim 14 , wherein the adsorbent material is selected from the group consisting of zeolites of types LTA, FAU, CHA, and ERI, and materials kinetically selective for NF 3  having pore openings ranging from approximately 3.6 angstroms to approximately 4.6 angstroms. 
     
     
       17. The method of  claim 16 , wherein the adsorbent material comprises a mixture of primarily CHA zeolite with a smaller amount of erionite. 
     
     
       18. A method for separating a mixture comprising nitrogen trifluoride (NF3) and carbon tetrafluoride (CF4), the method comprising:
 providing an adsorption column packed with an adsorbent material that that preferentially adsorbs NF 3  over CF 4 , wherein the adsorption column comprises a feed end and, opposite the feed end, a product end, wherein the feed end and the product end are initially closed; 
 opening the feed end of the adsorption column; 
 flowing a first volume of a feed gas comprising NF 3  and CF 4  into the feed end of the adsorption column until the adsorption column reaches a first pressure, whereby a fraction of the NF 3  of the first volume of the feed gas is adsorbed by the adsorption material; 
 closing the feed end of the adsorption column; 
 opening the product end, whereby a first product having a CF 4  concentration greater than the CF 4  concentration of the feed gas exits the product end of the adsorption column until the adsorption column reaches a second pressure less than the first pressure, and, after the adsorption column reaches the second pressure, a second product having a CF 4  concentration less than the CF 4  concentration of the feed gas exits the product end of the adsorption column until the adsorption column reaches a third pressure less than the second pressure. 
 
     
     
       19. The method of  claim 18 , further comprising flowing a second volume of the feed gas into the feed end of the adsorption column to maintain the second pressure within the adsorption column while the first product exits the product end of the adsorption column. 
     
     
       20. The method of  claim 19 , further comprising:
 opening the feed end of the adsorption column and closing the product end of the adsorption column; 
 flowing a third volume of a feed gas into the feed end of the adsorption column until the adsorption column reaches the first pressure, whereby a fraction of the NF3 of the third volume of the feed gas is adsorbed by the adsorption material; 
 closing the feed end and opening the product end, whereby a third product having a CF 4  concentration greater than the CF 4  concentration of the feed gas exits the product end of the adsorption column until the adsorption column reaches a second pressure less than the first pressure before closing the product end; 
 opening the feed end of the adsorption column; 
 flowing a portion of the second product gas into the feed end of the adsorption column to maintain the second pressure within the adsorption column while the third product exits the product end of the adsorption column; and 
 closing the feed end and opening the product end, whereby a fourth product having a CF 4  concentration greater than the CF 4  concentration of the feed gas exits the product end of the adsorption column until the adsorption column reaches the third pressure. 
 
     
     
       21. The method of  claim 18 , wherein the method does not comprise using a substantial volume of an inert carrier gas. 
     
     
       22. The method of  claim 18 , wherein the adsorbent material is selected from the group consisting of zeolites of types LTA, FAU, CHA, and ERI, and materials kinetically selective for NF 3  having pore openings ranging from approximately 3.6 angstroms to approximately 4.6 angstroms. 
     
     
       23. The method of  claim 22 , wherein the adsorbent material comprises a mixture of primarily CHA zeolite with a smaller amount of erionite.